That document is not a proposal, it is a problem statement. It goes without saying that IPv4 only clients - every single one of them - would need to be upgrade

That document is not a proposal, it is a problem statement. It goes without saying that IPv4 only clients - every single one of them - would need to be upgraded in some fashion to support a larger address space. The issue is how to do that while remaining incentive compatible. The only way to remain incentive compatible is to remain administratively compatible, and that is where IPv6 as presently constituted fails dramatically by requiring two independent network configurations to be maintained for the better part of a century, without giving anyone an incremental incentive to maintain the second one, leading to a hold out problem. The public switched phone network has gone through major upgrades and yet at no point did someone say we are going to throw out all your existing phone numbers and require you to get new ones, or require you to have two independent and incompatible phone numbers that you put on your business cards, have two phones on your desk, or a phone with a mode selection button depending on whether you wanted to call a new style phone number or an old style phone number. And that - from an administrative point of view - is the fundamental problem with the deployment of IPv6 as we know it. Dual stack now and for decades to come. Dual stack anything is not incentive compatible and should never have been done. The proper solution is single stack everything with capabilities that are dormant until they are deployed on a global level as part of the normal upgrade process in an administratively compatible fashion so that no large scale administrative intervention is required now or at any time in the future.